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    <meta charset="UTF-8" />
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    <title>Document</title>
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    <script>
      function getLastDayOfMonth(year, month) {
        const d = new Date(year, month, 0); //8
        return d.getDate();
      }
      const r = getLastDayOfMonth(2012, 9);
      console.log(r);
    </script>
  </body>
</html>
